1. Field of the Invention
The present invention is broadly concerned with a mobile reactor apparatus, and a corresponding method, which permits the on-site fumigation of grain silos and other enclosures with phosphine gas. More particularly, the invention pertains to such apparatus and methods which can achieve relatively high and substantially constant levels of phosphine gas in a silo in order to obtain maximum fumigation thereof, to this end, the invention contemplates a mobile unit supporting a reactor assembly enabling on-site production of phosphine gas directly adjacent a silo or the like so that phosphine may be continuously and directly fed into the silo without intermediate storage in containers.
2. Description of the Prior Art
Fumigation of stored agricultural products such as grain with phosphine gas is the foremost method of preventing insect damage. Typically, fumigation is achieved by introducing metal phosphide pellets or tablets directly into the grain to be fumigated. The metal phosphide reacts with ambient moisture in the air and grain, resulting in the production of phosphine and inert gases. Forced air circulation devices are often used to assist in distribution of the phosphine gas throughout the grain. Where the grain is stored in unsealed containers, a gas tight plastic sheet may be placed over the container to retain the fumigant gas.
A common fumigation problem encountered with prior practices is the inability to maintain a constant concentration of phosphine gas within the silo for an adequate time to achieve complete insect kill. It is known that for the most effective insect control, it is necessary to maintain specific relatively high concentrations of phosphine gas for known periods. However, with prior methods the release of phosphine gas is slow in the initial stages of treatment and the concentration of phosphine builds up only gradually. After a maximum is reached, the concentration of phosphine begins to decline. It is accordingly very difficult for the fumigator to ensure that the necessary phosphine concentration-time parameters are met to achieve complete insect kill. As a consequence, many fumigators tend to over treat grain with phosphine, which is not only wasteful but can present a hazard owing to the flammability of phosphine gas.
Batch processes for the generation of phosphine gas have been proposed in the past, see, e.g., Australian Published Application No. 73446/87. This reference describes a batch reactor for the hydrolysis of metal phosphides to obtain phosphine gas, the latter being stored in closed containers. Such containers can then be used on-site to deliver phosphine gas at a specific concentration throughout a selected period of fumigation. A significant drawback of this technique is the need to containerize the phosphine gas in pressure vessels with subsequent delivery of said vessels to the silo site.
There is accordingly a need in the art for an improved apparatus and method allowing the safe, continuous production of phosphine gas at the site of use so that a desired phosphine concentration/time fumigation can be achieved while avoiding the need to produce phosphine at a remote site with intermediate containerization of the phosphine.